To me, the audio sounded like "MURES antiqui sunt plurimi," There are very many ancient mice (like Horace's town mouse and country mouse, for example!).
I guess it's supposed to be MORES antiqui, there are many (or "very many"--how about "quite a lot of", in English) ancient customs.
"The ancient customs are very many"? What non-English brain thought up that abomination of a sentence? I tried "The old customs are very numerous", which is a little inaccurate, and still clunky. On reflection, I think this is definitely a case where the tried and tested "there is/are" applies: "There are very many old customs".
We've got to keep the plūrimī as the predicate adjective (the one that 'comes after are ') and antīquī as the attributive adjective (the one that 'already belongs' to mōrēs ).
How about, "The ancient customs are extremely numerous" ? (anything but "very many," I agree!)
I completely agree with you. The difference between adjectives used attributively or predicatively is an important one which is why Duo's literal translation of this sentence is correct. Still, this is a tricky issue because Duolingo doesn't provide any grammatical information and is essentialy simply an vocabulary training app.
But we're talking about the English translation, NOT the sentence structure of the Latin. We get it! We get why the words are in that order, but "There are very many ancient customs" is EXACTLY THE SAME MEANING as "the ancient customs are very many", but the latter is not how you actually speak English. So there is NO reason why the correct TRANSLATION of this cannot be "there are very many ancient customs". This is why people are annoyed at this. At least add BOTH translations as correct answers.
I suppose that they would want the sentence Sunt plūrimī mōrēs antīquī for your translation; i.e., with the verb "to be" (Sunt ) starting the sentence.
Notice that, in Duo's sentence, the subject (mōrēs antīquī) and its complement (the adj. plūrimī) are "separated" by the verb.
It seems like a minor difference to me, but ... perhaps it's 'different enough' ?
"Old habits are very many" I had... = error. What about "There are very many old habits". I get now that it ought to be "customs", but what about mores = character (sing.)? That would be something like "There is a lot of old character", as in when describing a classic sword and sandals flick?